It is well known that a directly radiating loudspeaker is unable to achieve high radiation efficiency at low frequencies. For example, a loudspeaker diaphragm with a diameter of 25 cm has a radiation efficiency of 0.7% at 50 Hz when mounted in an infinite baffle; this efficiency falls to about half when mounted in a cabinet (See the Newell and Holland reference cited in the Information Disclosure Statement). An “acoustic horn” or “waveguide” is defined, for purposes of this disclosure, as a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. An acoustic horn is designed for efficiently transferring sound waves from the particular source are transferred to the air.
Horns are some of the most commonly used tools in acoustics. They have been used for centuries for passively amplifying sounds. Horns are acoustic elements specially designed for maximum transmission of sound pressure, for example, in sound systems. Horn systems are capable of giving a closer approximation of musical reality.
Horns were the earliest form of acoustic amplification. An acoustic horn is an acoustic waveguide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximizing the efficiency with which sound waves from the particular source are transferred to the air. Horns do not use electricity. The problem with conventional acoustic horns is that amplification is limited.
FIG. 1 shows a prior art exponential horn. The acou stic horn is used to increase the overall efficiency of the driver. A horn effectively guides the motion of sound waves and thus substantially increases the sensitivity and effectiveness of sound radiation. A horn is a natural and powerful way to amplify sound. The horn may be viewed as an acoustic impedance transformer. When a diaphragm vibrates, pressure waves are created in front of it. This is the sound we hear. Coupling the motion of the diaphragm to the air is not an easy thing to do due to the very different densities of the vibrating diaphragm and air. This can be viewed as an impedance mismatch. Sound travels better in high density materials than in low density materials, and in a speaker system, the diaphragm is the high density (high impedance) medium and air is the low density (low impedance) medium. The horn assists the solid-air impedance transformation by acting as an intermediate transition medium. In other words, it creates a higher acoustic impedance for the transducer to work into, thus allowing more power to be transferred to the air. A contributor to the higher efficiency of horn loudspeakers is the fact that they are better matching acoustical impedances of the source of the sound and the so-called load (air). The higher pressure increases the external impedance—which is naturally low due to low air density—to (better) match the impedance of the source. In (conventional) direct radiating loudspeakers this mismatch leads to a lot of energy being converted to heat within the driver instead of into the sound wave.
A horn is defined as, for purposes of this disclosure, a tube whose cross-section increases exponentially or at least at a greater and greater rate. The narrow end is called the throat and the wide end is called the mouth. The transducer is placed at the throat. When the diaphragm moves near the throat, we have a high pressure with a small amplitude in a small area. As the pressure wave moves towards the mouth, the pressure decreases and the amplitude increases. Excellent natural efficient amplification.
Acoustic horns are used in sound systems primarily for two reasons: (1) high efficiency (and the resultant high acoustic output with low distortion) and (2) pattern/coverage control. The ideal horn should have constant directivity and coverage angle and provide a constant acoustic load to the driver at all frequencies in the designed operating range of the horn. Up to now these goals were for the most part met by designs based on exponential horn theory. The exponential horn was found to be particularly effective in providing good response right down to horn cutoff, specially for the hyperbolic-exponential designs.
Horns have very special properties, including lower distortion, faster transient response than conventional drivers, and are easier to drive at high sound pressure levels than conventional drivers.
The exponential horn has an acoustic loading property that allows the speaker driver to remain evenly balanced in output level over its frequency range. A major drawback is that the exponential horn allows for a narrowing of the radiation pattern as frequency increases, making for high frequency ‘beaming’ on axis and dull sound off axis. Another concern is that a throat of small diameter is needed for high efficiency at high frequencies but a larger throat is best for low frequencies.
FIG. 2 shows effects of horn shape on cut off frequency of a horn. Again, the horn is comprised of 3 main parts: (1) The throat: the part that is connected to the speaker. (2) The neck: which describes the length of the horn, and (3) The mouth or the bell: which describes the end part of the horn, “connected” to the air. The speaker is connected at the throat of the horn, and radiates sound at the mouth of the horn. All of these parts influence how will the horn affect the overall sound. The flare and mouth design, the phase and direction of the particle velocity at the mouth, will all have an impact on the sound quality and directivity of the horn. One of the main characteristics of the horn is its shape. The horn has a certain taper, which is determined by the cross section expansion rate. The cross section area is determined by a function of distance, from the throat of the horn along its axis. Some common horn profiles are as follows:
(a) Parabolic: Easy to design and construct, but poor impedance conversion.
(b) Conical: Easy to design and construct, but poor impedance conversion.
(c) Exponential: Good wide band impedance conversion, but some nonlinearity.
(d) Hyperbolic: Very good and high impedance conversion, but relative nonlinearity.
(e) Stepped: High impedance conversion. Nonlinearity depends on step resolution. This shape is not like the others. The horn is not growing in a smooth fashion, but in abrupt square steps (imagine a cube, then a larger cube, and so on. The speaker plays through these cubes).
The main advantage of horn loudspeakers is they are more efficient; they can typically produce 10 times (10 dB) more sound power than a cone speaker from a given amplifier output. Therefore horns are widely used in public address systems, megaphones, and sound systems for large venues like theaters, auditoriums, and sports stadiums. Their disadvantage is that their frequency response is more uneven because of resonance peaks, and horns have a cutoff frequency below which their response drops off. To achieve adequate response at bass frequencies horn speakers must be very large and cumbersome, so they are more often used for midrange and high frequencies. The first practical loudspeakers, introduced around the turn of the 20th century, were horn speakers. Due to the development in recent decades of cone loudspeakers which have a flatter frequency response, and the availability of inexpensive amplifier power, the use of horn speakers in high fidelity audio systems over the last decades has declined.
A horn is an acoustic transformer, changing high pressure and low volume at the throat to low pressure and high volume at the mouth. It does so by slowly expanding the cross section of the tube down which the sound wave travels, and it creates an acoustic load for the driver as if it had a very large diaphragm, dramatically raising its efficiency.
A horn loudspeaker is characterized by several numbers; the area of the small end known as the throat, the wide end known as the mouth, the distance from the throat traveling down the length of the horn toward the mouth, and the expansion curve of the cross section of the horn as sound travels that distance. In an exponential horn, this expansion is given by the initial throat area multiplied by the natural logarithm (e) raised by a power factor related to the distance down the horn and the lowest frequency. FIG. 1 shows these elements and the formula giving the desired cross-sectional area, which is proportional to (e) raised to the power of 4*π times X inches divided by λ, the wavelength of the cutoff frequency.
The following equations are commonly used to determine various parameters associated with an exponential horn:
      m    =                  ln        |                              S            L                                S            0                          |            L                  f      c        =                  m        ⁢                                  ⁢        c                    4        ⁢        π                        S      L        =                            (                      c                          2              ⁢                              f                c                                              )                2            π      Where fc is lower cut-off frequency, m is flare constant, SL is the mouth cross-sectional area and S0 is the throat area of the horn.
Every horn has a cutoff frequency. Below the cutoff frequency, a horn no longer works as an impedance transformer and the driver is then just pushing on the low impedance room air. At 60 hz, for example, one would need a horn dozens of feet in length. The cutoff frequency is determined by how rapidly the area increases, especially at the beginning of the horn where the pressure differentials are large. The more gradually the horn flares out, the lower the cutoff frequency will be.
One of the main disadvantage of a typical acoustic horn is that it is very large in dimensions which renders it unsuitable for it to be used with small scale modern electronic systems, such as smart phones, laptops, TVs, etc. In other words, conventional, prior art acoustic horn systems can not be miniaturized enough to be used with small electronic devices.
Miniaturization and integration of acoustic devices have been an important development in recent times. Consumer electronic devices, such as cellular phones, laptops, tablets, and the like with more features and capabilities are ubiquitous and are positioning to become audio entertainment centers. However, they exhibit severe audio deficiencies and pose many additional challenges to maintain the acoustic performance as enclosed acoustic volume size, power and membrane size are reduced significantly.
Recent consumer studies have indicated that people, in general, rate excellent sound quality as one of the most important features in an audio system and have less patience for poor sound quality. What is therefore needed in the art is an efficient way to propagate and amplify sounds over a broad frequency range in a small space. With the proliferation of handheld devices such as phones with small speakers, which are also used to play music, high tech games and the like, there is certainly a need for sound amplification with quality to catch up to the miniature size of the devices.